The production of lactic acid from carbohydrate-containing materials by fermentation is increasingly gaining in importance. However, other possibilities for the obtention of lactic acid via chemical transformations of reactants which derive from petrochemistry, such as for example the hydrolysis of lactonitrile, are also known to those skilled in the art. Lactic acid is an environmentally harmless intermediate for the production of cleaning agents, liquid soaps, scale removers and textile additives. Interest in lactic acid has further increased recently since the polymeric form of lactic acid, polylactide, is compostable. Polylactide or polylactic acid is used as a biologically degradable and well tolerated plastic in the food industry, in cosmetics and in medical technology. There is particular interest in carrier bags made of compostable polylactic acid films, since carrier bags made of conventional plastic are not degraded in the environment and are therefore a major environmental pollutant. In contrast, plastic carrier bags made of polylactic acid are biologically degradable and thus an environmentally harmless alternative to carrier bags made of conventional plastic.
Lactic acid occurs in two diastereoisomeric forms, as L(+)- and D(−)-lactic acid. The starting material for the fermentative production of lactic acid is a carbohydrate-containing material, which is transformed into lactic acid by treatment with microorganisms suitable for this. Suitable bacteria for this are for example lactic acid bacteria from the genus of the Lactobacillaceae, but also microorganisms from the genus of the Saccharomyces or Rhizopus. Depending on the genus of the microorganisms used, one or both of the aforesaid diastereoisomeric forms of lactic acid are obtained.
Decisive for the industrial utilization of lactic acid which is generated by fermentation of carbohydrate-containing substrates by various microorganisms is the economy and efficiency of the separation and purification of the lactic acid from these aqueous fermentation solutions, which apart from the lactic acid or the lactic acid salts also contain further organic acids, other side products of the fermentation, microorganisms and components thereof and residues of the substrates, such as sugar.
These impurities interfere in a subsequent polymerization of the lactic acid to polylactic acid and thus in the production of biologically degradable plastics. It has long been known, and for example follows from J. Dahlmann et al, British Polymer Journal, Bd. 23 (1990), p. 235. 240, that extremely pure monomer has to be used in order to achieve a high degree of polymerization of the lactic acid. In order to achieve the desired high degree of polymerization, the lactic acid after passing through purification must have a concentration of 80 wt. %.
From the literature, a large number of methods relating to the purification of lactic acid are known.
For example, the teaching in some patents is to use distillation for the purification of lactic acid from aqueous solutions. Such a process is used in EP 0986532 B2. In DE 10 2007 045 701 B3, a combined extraction with linear n-trioctylamine (TOA) and a distillation is disclosed. Further possibilities known in the literature are electrodialysis or esterification with an alcohol, after which likewise distillation and then hydrolysis of the ester formed are performed. These processes are extremely cost-intensive. Moreover, distillation has the disadvantage that a part of the carbohydrates is always also simultaneously extracted, which results in worsening of the yield of the whole process and complicates the isolation of the product.
Processes using calcium hydroxide and sulfuric acid, wherein gypsum is formed in large quantities as a side product, are also known. In this connection it was moreover found that lactic acid can for example be isolated from a fermentation broth acidified with sulfuric acid, which as well as free lactic acid also contains ammonium and sulfate ions, by chromatographic methods. DE 69815369 T2 for example inter alia describes the separation of lactic acid from aqueous mixtures by adsorption onto a solid adsorbent, and preferably a solid adsorbent which adsorbs lactic acid versus lactate is used here.
In particular, according to the above document, weak anion exchangers are possible for the isolation of lactic acid. Further, DE 10 2009 019 248 A1 describes chromatographic methods for the purification of organic acids, especially of lactic acid, by the use of simulated moving bed chromatography.
WO 2006/124633 A1 describes a process for the production of ammonium lactate by fermentation. In the fermentation, the ammonium salt of lactic acid is formed, which can be separated from the fermentation solution e.g. by extraction. The ammonium salt can very easily be cleaved in a subsequent step with weak acids or carbon dioxide. The free lactic acid is thereby obtained, which can then be purified for example by distillation.
A disadvantage of many processes is that additional substances are added to the process, which must no longer be contained in the product or whereof traces in the target product can result in limitations in the quality and usability of the product. Thus the purification processes from the state of the art often result in inadequate quality of the purified lactic acid and polymerization to polylactic acid is not possible to the desired extent. The practical implementation of the processes is also sometimes associated with considerable technical and energy expenditure. The quality of the purified lactic acid often first becomes apparent when polylactic acid is to be produced.